如果条件为真，则在MySQL存储过程中运行查询

Question

我正在使用MySQL数据库并尝试创建一个存储过程。如何才能使其在query1的结果没有记录时执行不同的查询？

这是我到目前为止所知道的：

/* CREATE DB */
CREATE DATABASE mydata;
use mydata;
/* TABLE */
CREATE TABLE mydata (
ID BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
Name                        VARCHAR(255) NOT NULL,
Value                       VARCHAR(255) NOT NULL
) ENGINE=InnoDB;
INSERT INTO mydata (Name, Value) VALUES ("testname", "testvalue");

/* STORED PROCEDURE */
delimiter //
CREATE PROCEDURE myproc(IN myTable VARCHAR(255), 
                        IN myValue VARCHAR(255), 
                        IN myValueTwo VARCHAR(255))
BEGIN
    SET @iTable=myTable;
    SET @iValue=myValue;
    SET @iValueTwo=myValueTwo;

    SET @query = CONCAT('SELECT Name FROM ', @iTable, 
        ' WHERE Value="', @iValue, '"');
    SET @querytwo = CONCAT('SELECT Name FROM ', @iTable, 
        ' WHERE Value="', @iValueTwo, '"');
    PREPARE QUERY FROM @query;
    EXECUTE QUERY;

END //
delimiter ;

/* CALL */
call myproc("mydata", "testvalue", "");

我想运行一个查询，并仅在第一个查询没有行的情况下执行第二个查询。做这件事最好的方法是什么？

Answer 1

这需要一些工作，但我做了足够的调整。代码的问题与逻辑无关，而与MySQL存储过程语言本身有关。在执行动态SQL时，它存在作用域问题。

我所做的是创建一个临时表，并将返回值存放在其中

以下是加载的一些示例数据

mysql> drop database if exists user391986;
Query OK, 1 row affected (0.08 sec)

mysql> create database user391986;
Query OK, 1 row affected (0.00 sec)

mysql> use user391986
Database changed
mysql> CREATE TABLE mytable (
    -> ID BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    -> Name VARCHAR(255) NOT NULL,
    -> Value VARCHAR(255) NOT NULL
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.11 sec)

mysql> INSERT INTO mytable (Name,Value) VALUES
    -> ('rolando','edge'),('pamela','washington'),
    -> ('dominique','wilkins'),('diamond','cutter');
Query OK, 4 rows affected (0.06 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * from mytable;
+----+-----------+------------+
| ID | Name      | Value      |
+----+-----------+------------+
|  1 | rolando   | edge       |
|  2 | pamela    | washington |
|  3 | dominique | wilkins    |
|  4 | diamond   | cutter     |
+----+-----------+------------+
4 rows in set (0.00 sec)

mysql>

下面是为捕获临时表中的返回值而调整的存储过程

mysql> delimiter //
mysql> CREATE PROCEDURE myproc(IN myTable VARCHAR(255), IN myValue VARCHAR(255), IN myValueTwo VARCHAR(255))
    -> BEGIN
    ->     DECLARE foundcount INT;
    ->     DECLARE retval VARCHAR(255);
    ->
    ->     SET @iTable=myTable;
    ->     SET @iValue=myValue;
    ->     SET @iValueTwo=myValueTwo;
    ->
    ->     CREATE TEMPORARY TABLE IF NOT EXISTS mynumber (rv VARCHAR(255)) ENGINE=MEMORY;
    ->     DELETE FROM mynumber;
    ->
    ->     SET retval = 'nothing retrieved';
    ->     SET @query = CONCAT('INSERT INTO mynumber SELECT Name FROM ', @iTable, ' WHERE Value=''', @iValue, '''');
    ->     PREPARE QUERY FROM @query;
    ->     EXECUTE QUERY;
    ->     DEALLOCATE PREPARE QUERY;
    ->     SELECT COUNT(1) INTO foundcount FROM mynumber;
    ->     IF foundcount = 0 THEN
    ->         SET @querytwo = CONCAT('INSERT INTO mynumber SELECT Name FROM ', @iTable, ' WHERE Value=''', @iValueTwo, '''');
    ->         PREPARE QUERY FROM @querytwo;
    ->         EXECUTE QUERY;
    ->         DEALLOCATE PREPARE QUERY;
    ->     END IF;
    ->     SELECT COUNT(1) INTO foundcount FROM mynumber;
    ->     IF foundcount > 0 THEN
    ->         SELECT rv INTO retval FROM mynumber;
    ->     END IF;
    ->     SELECT retval;
    ->
    -> END //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;
mysql>

好的，我调用了存储过程三次。第一个一无所获。第二个获取第二个值。第三个获取第一个值。

mysql> CALL myproc('mytable','pamela','diamond');
+-------------------+
| retval            |
+-------------------+
| nothing retrieved |
+-------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

mysql> CALL myproc('mytable','pamela','wilkins');
+-----------+
| retval    |
+-----------+
| dominique |
+-----------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> CALL myproc('mytable','edge','wilkins');
+---------+
| retval  |
+---------+
| rolando |
+---------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

mysql>

试一试吧！

Answer 2

在Sql Server中，您可以在临时表中运行结果，然后检查临时表中是否有如下所示的行：

declare @numberResults int = 0

create table #MyTempTable(...)

insert #MyTempTable
sp_executesql Query

set @numberResults = (select count(*) from #MyTempTable)

if @numberResults = 0
 begin
  sp_executesql secondQuery
 end

Answer 3

在mysql中，可以使用found_rows()内置过程，如下所示：

el@apollo:~$ mysql -u root -p
Enter password: 
mysql> use your_database;
Database changed
mysql> select id from problems;
+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
|  4 |
+----+
4 rows in set (0.00 sec)

mysql> select found_rows();
+--------------+
| found_rows() |
+--------------+
|            4 |
+--------------+
1 row in set (0.00 sec)

在mssql中，您可以使用值@@rowcount。然后，仅当第一个查询未返回任何行时，才能运行第二个查询：

EXECUTE QUERY;

IF @@rowcount = 0
BEGIN
     PREPARE QUERY FROM @querytwo;     
     EXECUTE QUERY;
END